1. Field of the Invention
This invention relates to a heat-resistant adhesive especially suitable for the bonding of a semiconductor chip to a lead frame and an adhesive member using the adhesive.
More particularly, this invention relates to a heat-resistant adhesive suitable for use in a semiconductor package, especially a package of LOC (lead on chip) structure, which is capable of preventing or minimizing the occurrence of cracks when solder is subjected to reflow when the package has absorbed moisture.
2. Description of the Related Art
The semiconductor package is fabricated by connecting a lead frame to a semiconductor chip using an adhesive or resonance solder and electrically connecting electrodes of the chip to the lead frame, followed by sealing the entire structure into a molded form as illustrated in FIGS. 1 thru 3.
In general, a conventional package has a structure as shown in FIG. 3 where the chip is mounted on a tab (die pad) of the lead frame. The connection between the lead frame and the chip is made by Au--Si resonance solder or a thermosetting epoxy adhesive (a die bonding agent).
However, as the integration degree of the package has become higher and a size of a chip to be mounted therein has become larger, the percentage of the volume occupied by the chip has become higher to such an extent that the chip cannot completely be encased in a structure as shown in FIG. 3 where the chip is mounted on the tab of the lead frame. To solve this problem, a package having no tab such as shown in FIG. 1 or FIG. 2 has been proposed, for example, in U.S. Pat. No. 5,140,404, Japanese Patent Application Publications (KOKAI) 61-218139 and 61-241959, U.S. Pat. No. 4,862,245, and Japanese Patent Application Publications (KOKAI) 2-36542 and 4-318962. The package structures of FIGS. 1 and 2 are called LOC (lead on chip) and COL (chip on lead), respectively. In these packages, the connection between the lead frame and the chip is made by a thermosetting adhesive or a heat-resistant hot melt.
With the structures as shown in FIGS. 1 and 2, it is the trend that as the package size is becoming smaller and thinner and the percentage of the occupation by the chip is becoming higher, the thickness of the sealing material is becoming thinner accordingly. Therefore, moisture absorbed by the adhesive or the sealing material would more readily cause cracks in the package when it evaporates or expands.
To solve this problem, some attempts to improve the sealing materials and/or the adhesives employable for the package have been proposed. For example, with respect to the sealing materials, lowering of moisture absorption and improvement of mechanical strength have been discussed (Japanese Patent Application Publication (KOKAI) 5-67703). With respect to the adhesives, it has been proposed to reduce the moisture absorption or to divide an adhesive member into small pieces to promote escape of vapor during reflow in the soldering (Japanese Patent Application Publication (KOKAI) 3-109757). However, there have been no proposals directed towards modifying the properties of the adhesive to be employed for the LOC package.
There has been another attempt which intentionally allows the solvent to remain with a view to lowering a bonding temperature, lowering the viscosity of the resin (Japanese Patent Application Publication (KOKAI) 3-64386) or allowing a solvent to remain to improve the fluidity and increase the adhesion to lower the possibility of a leak current between inner leads (Japanese Patent Application Publication (KOKAI) 2-36542).